What was surprising, at least to this Trump campaign observer, was the campaign's response to this revelation.

Within an hour after being alerted to Nunberg's Facebook's posts, a Trump campaign spokesman told Business Insider that Nunberg will be " terminated immediately" once the posts' authenticity is verified.

This reaction, of course, is the reaction that might be expected from any serious candidate for the US presidency.

Until now, however, Trump has not behaved like a serious candidate for the US presidency.

Rather, he has behaved like a schoolyard bully who feels entitled to say and do whatever he likes and attack anyone who criticizes him for any reason, no matter how valid such criticisms might seem.

During his rocket ride to the top of the Republican polls, Trump has seemingly gleefully insulted war heroes, major US corporations, other candidates, Mexico and Mexicans, his former wife, influential news organizations, his political party, and other constituencies that most serious candidates for the US presidency usually strive not to insult or offend.

So it seemed reasonable to expect that Trump's response to this latest revelation might be to shoot the messenger (in this case, Business Insider).

But that is not how Trump has responded.

Rather, days after adopting a new, more gracious tone toward his Republican opponents with respect to the upcoming debate, Trump is firing the adviser.

This move, along with what seems to be other modestly more "presidential" behavior on Trump's part in recent days, suggests that he is now taking his campaign for president more seriously.

To be clear: I have never doubted that Trump is enjoying his campaign. In fact, it struck me that he has been having the time of his life. But now, given his startling and sustained position atop the polls, it seems like Trump might be thinking that he could actually make a serious go of this thing. And, thus, that he should begin to behave — modestly — more presidential.